Band pass filter



2 Sheets-Sheet l Zmventor Nou mA/Jil b' :Uma/@aww V. D. LANDON BAND PASS FILTER Filed Nov. 28, 1936 Juy 25, 1939. v. D. LANDON BAND PAss FILTER Filed Nov. 28, 1936 2 sheets-sheet 2 3 nventor Gttorneg Patented Jul'y 25, QS

PATENT VOFFICIE vBAND PASS FILTER Vernon Landon, Haddonfield, N. J., assignor to RadioA Corporation of America, a corporation of Delaware Application November 28, 1936, Serial No. 113,157

" Y Y1s claims., (ci. irs- 44) 'The present invention relates lto band pass filters for high'frequency radio signaling circuits and the like, such `as are found in radio frequency Aand intermediatefrequericy amplifiers of radio receiving systems'.

The invention relates more particularly to intermediate or constant frequency amplifiers, and has for its object to provide an improved band pass filter network which may provide two points of signal attenuation adjacent to the pass band of the filter, to provide sharp cut-olf of undesired signals and to provide a relatively high input and output impedance, whereby the filter is adapted to be used asV an interstage coupling means in connection with high gain, high impedance amplifier tubes. 1 Y

In,v oneof its. aspects, the inventioncontemplates an improvement upon the circuits shown in patents Ito VMason 1,967,249 and 1,967,250 to obviate the necessity for utilizing a crystalas the lter means, whereby the cost of the filter is reduced. 1

A form of band pass filter to which the invention is readily adapted is that known as an M-derived band pass filter withy mid-shunt termination. It isan .object ofthe presentinvention to provide means for compensating the positive resistvance Yin the attenuatingcircuits by asingle circuit element connected between 4two points on the lter network and to'improve the input andeutput impedance of a filter network ofthe character above referredto-.- y g if Vv. It is a furtherg obj ect of thepresentinvention Vto provideqa high impedance'bandpass lter,

with the same attenuation and cut-off characteristics A as a dissipationless filter, without using crystal filter elements therein. l It is a still further object of the present inventionto provide a practicalcircuit for, obtaining infinite attenuation aty a vplurality of rejection frequencies in a .band .passcoupling network in `connection Withhigh gain amplifiertubes, with- -outvresorting' to complicatedmeans for effecting resistance cancellation therein.

Y 'I'heinventionwill be understood from the fol- -lowing description,'when considered in connection with theaccompan'ying drawings, and its scope will be pointed out in the appended claims.

In the drawings,- Y

Figure 1 is a schematic circuit diagram of-fa portion ofa high frequency orr intermediatefrequency amplifier embodying the invention,

Figures 2 and 3 are curves illustrating certain BOde 2,002,216,

operating characteristics of the system shown in Fig. `1, and

Figures 4,15 and 6 are schematic circuit diagrams of modifications of the invention shown in Fig. 1. 'Y

Referringto Fig. 1, a plurality of vhigh gain screen grid amplifier tubes IIl, II and I2 are corinected'in cascade relation to provide a multistage amplifier, having input leads I3 and output leads 14.! AThe amplier'tubes 'are coupled, in ac- -cordancev with the invention, by means of filter networks indicated generally at I5 and I6, the formerbeing interposed between an output circuit I'I' and an input circuit I8 of the iirst and second stage tubes I0 and II, respectively. The

`second lter network I6 provides coupling means forthe output circuit I9 of the second stage tube 'II Yand the input circuit 20 of the third stage tube I2.- Suitable output coupling means 2| and 22 are provided for the output circuits I1 and I9 and similar coupling means 23 and 24 are pro- -knownv 'The' low potential side of the circuit is indicated by a terminal D through which the capacitors C1 and C1 are connected to the ground lead 29 'and chassis ground 30 of the amplifier. The capacities represented by C1 and C1 may in part be supplied by fixed or variable capacitors 'and may also be provided by the Vinternal tube capacities connected' therewith. With high inductance elements; these vprovide relatively high input and' output Vimpedances for the filter network in connection with the tubes.

- Filter networks of this character are referred to in the patent to Z obel"'1,538,964 issued May 26,1925 and are also'referred to in the patent to issued May 21, 1935. The bridged-'l'. type of network shown includes equal inductances L1 and L1 and the capacitors C1 and Ci'fin shuntrather than in series whereby the:

impedance of the network at the inputterminal Aand theoutput terminal B is raised to a value suitablefor use between high impedance amplifier devices of the type shown at I0 and II.

Furthermore, the shunt circuit orA stem of the T network comprises two tuned series resonant circuits comprising capacitors C2 andY C3 and series inductances L2 and L3. These are provided by inductors having variable magnetite core tuning as indicated, whereby the resonance Vfrequency of the circuit is variable in conjunction with xed capacitors at C andC's. The inductances L1 and L1 preferably are also provided by variable magnetite cores as indicated. Adjustable core inductances are known and extensively used, and as shown, may be rof any suitable type. Inthe filter network shown, the use Vof variable inductance devices, particularly with -Y-magnetite cores, vnot. w i

' '.anceatther-frequency F1, rwhile the Vcncult CaLs 15 only permits variable ,tuning i to an `accur ate ferequency response, with permanent adjustment, but

permits,V the inductance ofgsuhcdevices toghe 1 relatively high as is desirable to permit theuuse Vof series tuning capacitors having relatively'ilow capacity., L Y. g p Y It will be noted that two series tuned circuits .connected in parallel provide thestemoffthe TY Ynet w'ork, These circuitsaretuned to differing frequencies, on opposite sides of4 the pass band. V'I hat is, each is tunedn to a frequency adjacent to and outside the pass. band, for attenuating signals adjacent in frequency tothe cut-off fre- `quency of' the `filter networkat each end of the pass band. They are, therefore, dissipative vre1- actan'ces in the stem of the .TV Vstructure of the network,v y Y, t

Itrhas been found that a high degree of attenuationmay be provided by this arrangement Y and that the resistance of the attenuation circuits may both be cancelledby thesingle resistor Vmeans 'R' connected between the input, terminal v ,KA and the output terminal Blofthe lterfnet- .-work, thus', eliminating the necessity for an ad- '.ditional stage for jattenuatingV bothsides,of the passv bandas has heretofore been considered necessary.

v The two meshesnLl-Clr are each tuned toftheV meanv frequency of the 'passy bandi Themesh Lz-Cz is tuned to one attenuation peakV outside 'Y thepass band, and'v Lg-Ca yis ituned tofthefpther attenuation` peak.L ForeXample, as'indicated at n frequency isindicated at, AF. f Considering La -Cz and. Ls--Cs in parallel, it will;beseenathat-thete are two frequencies at which the impedance YofV Y the stern of the T network may assume Athe proper lvaluef Yfor infinite attenuation.; The response characteristic is indicated jby the curv e"Sii-.inY vFig. f v 'Y Y It should bentedtnatths performance-curve has `Vhigh Y attenuation immediately on Veither side ',of-thepass band but that theattenuationis; re-

sponding elements are ,similarlygdesignatedr V.f'VI'vhe 3 lterpnetwork] B imayjbe tuned the samemanducedV at frequencies farther removed from the pass band; Accordinglynin orderY tomobtain va 'nearer approach Vto-anideal response curve, the

filtersection or-network-l is provided between `the amplifierv tubes IlVV and Alrlhnflhis isthe `same as the filterV sectionfornetwork'rpl Vandv correfner but for attenua/tion at peaks farther removed from thiiassabanda; ,Illfliav 3 theresponsechar-V acteristic ofthe filter Iis indicatedrby thecurve acent to and over a rangeof frequencies some- A reject-or circuits is reduced v-to detuned frorrrthe'mean-pass band frequency to f indicatedlby. the curve 35;'in' Fig. 3.

what widely removed from the pass band is effectively prevented through the cooperation of vthe two filter sections and the tuned coupled circuits in conjunction with the coupling transformer 25.

' In this network, the T-network between points A and B causes a voltage at B, 180 out of phase at the rejection frequencies'Lwlfiich voltage is cancelled by a' voltage drop through the resistor R at the point B. This is accomplished with a Vpair '10 of dissipative resonant circuits in an M-derived .r bandpass lter, with but one'resistor between points A and B. e Y ,Inthesecircuits Cz-Lz provide a low impedprovides-a lowimpedance at F2. The parallel combinationresonates at the frequency F, as shown in AFig'Zj thereby preventing attenuation at-tl1e` mean frequency of the pass band. With the resistor B. properlyadjusted, the aifltenuation vat the `frequencies yF1 vand F2 lis e made "subs-tan- Vtiallyinfinite. Y'Ifh'e response characteristic of the rejector circuit CzLzlis indicated bythe curve '40 and.I that of the circuit 'C3L3 is the curve Min Fig. 2.y v l In Figs. ll and 4 thel proper value for R is given Y by the formula.

indicated by ,I

, `R. in'which R2 is the 'effective "seriesfresistancefof' so LzCz or LsCa at resonance. jIn"Fig, 5-the proper value forrR is given by thekformiula f whichY Relaisthe effective series y-resistance'f-lof 1 jLzCz at resonance. adjustedwhen the effective resistancel-of.the substantially zero or minimum value.V f

Inl the filter. circuits -I 5 application andi-utilizationv of!A the vsignal. are aparalleledby thewterminalcapacitors C1' andai-Ci;

'1412. ,I il 2Z Y u, Ain'whichlRz isftheVVA effective; series resistance of V L2C2oriLsCalat-resonance.-In'Figple the proper `value for R .is given by.y theyformula `1'\.=4R.2, in

In any case, R.y is properlyV and. mit win be noted that the terminal resistance and the points of t This change fromseries to .parallelV resonant ci'r- 'Y cuitsl in the vinput .and :output elements 2 of the Lrl filter network 'accomplishes'.a desired increase-in impedance level, `and the circuits terminate-J in shown..V 'While-v these Ycircuitsvv are not considered vto Vbe conventionalf-itvhas been found that @they operate to provide the -desired responsefi'c'ur've,- particularly when lthefcircuits L1C1 are' slightly obtain a Lflat` ytransmission inthe pass Referring anew., vto-A the` I circuits shown sin.

4 1and;5, in case, that the plate requirementsare elements LiCL may be interchanged as indicated,

band, asA

toV providethe inductanceelements',L1,in shunt across the filter circuit-tand,,sincethese ,elements are of relatively high' inductance, thedfiighlim- Y pedance input characteristic ofthe lteris main- 7b tained. fiFOrathesrpurpose ofreadilycomparing-'the Vcircuits of Figs. ,1; iandfirthe same,re'ferenc'ze charance L1, at the input circuit is yin series with the anode circuit, Ifl of thertube lll in Fig. and the inductance in the input side.of the networkof Fig. 4 is ina similar positionto convey the plate current in lieu of a resistor ory other coupling device.

""'at' transmission in the pass bandit is .desirable ming. 5 'the 1'tr circu1t is further modified to provideI the rejector circuits CzLz-andCaLc in series in the filter network, while the compensating resistor AR is connectedv in the low potential side of the filter network in series with both irlductances or inductors L1. This circuit, like.that.

of. Fg. 4, has the advantage that one oftheinductors is arranged to carry the plate current of the tube to which the network is coupled. Furthermore, this circuit permits the tube capacity to be vused asl partof the filter input capacity,-

Whereas in Fig. l4 the filter does not-call for a terminal shunt capacity. l Therefore the circuit of Fig. l is useful lonly when the filter capacity@ V`C1, for` example, is less than the tube capacity -by arelatively wide margin. 'I'his is a ,bridged-T..

structur in which the arms are similarreactanges, as' in Fig. 1 but with the bridging im- Y pedance provided by o ne or more dissipative "parallel resonant circuits, while the resistance R islocated in the stem ofrv theT structure. 'Viewed in one way, the dissipation in the paral-r lel resonant circuit o r circuits maybe said to be subs ta ntiallyA nullified when R isrof sufiicient re. Ysistancefto provide a voltage equal to the impedancedrop`in the rejector circuits at the rejection frequency. l L

'vlnfconjunction with the method of detuning oneor more of the circuitsmL1C1 to provide a also "to concentrate at vone end of the filter network; thedamping or 4resistance by means of a single terminal resistor such as the resistor in: Fig. 5 .1' This may be adjusted untilthe proper. Vamountof dampingis obtained and this is detrmined bythe fact'that the shoulders of they pass band frequency characteristic have less tendency to droop when theterminal resistance is.con centrated at `one end of the iilter network.

LLikewisein the circuit of the resistors :2| and l23 are preferably Widely direrent in value,

23 being high and providing :substantially no load, whereby the loading `of the filter is latone end and provided by the resistor 2 I Referring now to vFig. 6, a. filter networklris shown in which the rejector circuit of Fig. 1 is modified to form a bridged T structure within the. T"structu`re of the filter network which includes Y .the input `shunt capacity 4l as C1, and series in-v ductance 49 -as L1 yat the input end, and the output series inductance 50 as L1 and the shunt capacityl as C1 atwthe output end. The rejector circuit comprises a dissipative reactance in the stem of the T having the capacitor sections 5 as in each arm of the T and providing reactances 'of opposite sign. The bridging impedance is the resistor 54 having approximately four times the resistance of the stem reactorV 52.

'If the capacitors 5| are each made equal to one half of the capacity C2 of Figs. 1, 4 and 5, and the inductor 52 made equal to the inductor L2, the filter network is then elfective to attenuate one side of the pass band, for example, at the frequency .F1 of Fig..3, while the resistance of the rejection circuit including theinductor. 52`V is effectively reduced to permit attenuation at the frequency1F1 to be, substantially infinite.-

.IIlfFig 6 the Vinfinite attenuation occurringa the resonant frequency. of. L2C2 can be easily (E) 4K2 equals 12R;

The power 'dissipated in is I2Rz. The total power dissipated is 2I2R2.-

Hence the 'effective impedance to ground from-` point 55 is 2122. The input voltage equals I2R2. The voltage drop across 4R2 equals v 4R; equals 2IR2 The voltage' drop across 4R2 being equal vto the 25 effect input voltage', thefpoint 5l Vis at4 ground poten'- tial. lleconnecting'/coil L1 50) hasY no 'since the voltageappliedto it isg`zlero.

Hence Since the currentsin the'two halves*l offCz are V not'exactly equal, the exact frequency of infinite attenuationis slightly removed from the resonant frequencyl of LzCji.l

Frorn the foregoing.descriptionV it will Vloe that advantage may be takenof the use of4r band. pass lters in high frequency ampliers, particu. larly intermediate frequency amplifiers as coupling unitsbetween high impedance circuits and that a plurality of rejection. frequency points may i be introduced into the response characteristic of.

the amplierby providing a vsuitable, number of`- lter coupled a'in'plinerstages, with orV without thewsirnple'addition of av pair `of tuned coupled circuits to vextendthe attenuation range on each side of. the band pass characteristic. i

The filter networks shownv and described. have the advantage thattthey maybe terminated by unequal resistances with 4'one resistance lumped at either end of the filter. thereby tocompensate for undesirable lpeaks in the response characteristic and to 4provide a substantially' atfresponse over the pass lallgeof. thefllter. A'

Furthermore, theffltei"r "networksfshown described have the acivalntageA tharafpiurauty of4 rejection circuitsimay be neutralize'd'as to theirl resistancewhereby a high degree of attenuation is attainedby the use of a singleresistor in theV circuit arranged inan .opposite branch to the t lter ybranch containing the rejector circuits.

I claim as my invention:

1. In a band passv signal filter network having atleast two branches, the combination of a pair ofv dissipative resonant circuits'in one branch Vof the network providing two frequency points of high signal attenuationV on opposite sides of the pass b and of the iilte'r, and a single resistor havin'g substantially no frequency characteristic connected in another branch of the network to improve the attenuation at both of' said frel quency points, said resistor having a resistance value such that the eiective resistance of each of said resonant circuits is thereby 'reduced to a minimum value.

2. In a band pass signal filter network having at least two branches,`the combination of a pair of .dissipative resonant circuits in oneV branch ofl bridged-T structure having two arms and a stem, a dissipative reactance in the stem of the T structure, means for causing the arms of the T to be reactive and equal to each other, and of such a. reactance value when used in parallel as to resonate with the stem of the T to a rejection frequency, means providing a; bridging impedance for the T structure having a. resistance of substantially four times the resistance of the resonant circuit provided by the stem of the T and two reactive arms, and means providing acceptor circuits at the input and output ends of 5 said rejector circuit.

VERNON D. LANDON. 

